U.S. patent number 3,936,878 [Application Number 05/428,601] was granted by the patent office on 1976-02-03 for disc interface location.
This patent grant is currently assigned to International Business Machines Corporation. Invention is credited to Walter R. Chrysler.
United States Patent |
3,936,878 |
Chrysler |
February 3, 1976 |
Disc interface location
Abstract
A disc storage file includes a multiplicity of closely spaced
continuously rotating flexible or floppy discs associated with an
accessing assembly for spreading the discs apart at randomly
selected positions to accommodate a magnetic head. Apparatus for
locating the disc interface (gap) to be accessed includes a pair of
offset photocells which are light coupled to the disc gaps. The
light applied to the photocells is guided in offset channels of a
shroud and shaped congruent to the photocell surfaces by offset
fiber optic shaping arrays. The offset arrays and associated
electronics allow for accurate counting of gaps in the presence of
fluctuating disc motion in the axial direction.
Inventors: |
Chrysler; Walter R. (Pleasant
Valley, NY) |
Assignee: |
International Business Machines
Corporation (Armonk, NY)
|
Family
ID: |
23699616 |
Appl.
No.: |
05/428,601 |
Filed: |
December 26, 1973 |
Current U.S.
Class: |
360/98.02;
360/71; 250/227.28; 250/570 |
Current CPC
Class: |
G06M
1/101 (20130101); G06M 9/00 (20130101) |
Current International
Class: |
G06M
9/00 (20060101); G06M 1/00 (20060101); G06M
1/10 (20060101); G11B 017/22 (); G11B 005/016 ();
G11B 025/04 () |
Field of
Search: |
;360/99,98,69,70,72,71
;340/146.3,174.1C ;250/227,570 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Eddleman; Alfred H.
Attorney, Agent or Firm: Lieber; Robert
Claims
What is claimed is:
1. In a rotating disc storage file comprising multiple spaced
flexible discs and access position sensing apparatus movable
relative to edges of said discs for detecting spaces between said
discs and for providing signals which can be used to accurately
count said spaces, improved sensing apparatus comprising:
a source of light for illuminating said spaces;
a pair of photocells; and
a pair of offset fiber optic shaping arrays movably positioned
between said discs and said photocells for conveying light from
said spaces to respective said photocells in a form congruent to
the shape of said photocells; said arrays having line-shaped light
receiving ends arranged in parallel lines offset from each other in
the direction of the axis of rotation of the discs by an offset
spacing distance less than the width of a disc space.
2. Sensing apparatus according to claim 1 including:
logic circuit means responsive to signals developed by said
photocells for developing count increment and count decrement
signal functions capable of being used to accurately count said
disc spaces even when edges of said discs are in transitory motion
relative to said fiber optic arrays.
3. Sensing apparatus in accordance with claim 2 wherein said logic
circuit means includes:
plural voltage comparator circuits coupled to said photocells for
developing signals useful to indicate the instantaneous direction
of movement of said light receiving ends of said fiber array
relative to said disc spaces and the displacement of the center of
said light receiving ends relative to the closest said disc space;
and
logic circuits coupled to said comparator circuits for utilizing
said directional and displacement signals to develop said count
increment and count decrement signals; said signals being useful to
control accurate counting of said spaces when the disc edges are in
transitory motion relative to said light receiving ends of said
fiber array.
4. In a rotating disc storage file having multiple coaxially
mounted flexible discs in an ordered axial sequence, sensing
apparatus movable relative to said discs for detecting accessible
storage spaces between said discs and bidirectional counting
circuitry for providing cumulative electrical count indications
corresponding to the instantaneous position of said sensing
apparatus relative to said disc spaces, improved counting apparatus
comprising:
first logic circuit means for utilizing signals provided by said
sensing apparatus to provide directional signal indications
representative of the instantaneous direction of movement of said
sensing apparatus relative to said discs;
a cumulatively conditionable bidirectional counter; and
second logic circuit means coupled between said first logic circuit
means and said counter for utilizing said directional signals to
develop increment and decrement inputs to said counter having
one-to-one correspondence to the said disc spaces traversed by said
sensing apparatus with allowance for transitory motion of said disc
edges relative to said sensing apparatus.
5. In combination with a rotating storage file, of floppy type
storage disks separated by smaller diameter spacer disks which form
ring-shaped open spaces between successive storage disks
insufficient for transducing access --wherein said disks have
transitory components of motion relative to said spaces and are
required to be separated at a selected said space for transducing
acess-- space locating apparatus comprising:
means for emitting radiant energy into said open spaces;
and apparatus electrically responsive to said energy separated from
said means by said file and movable relative to edges of said disks
and spaces in order to alternately receive and not receive radiant
energy from said emitting means depending respectively upon whether
a space or disk intervenes between said means and said apparatus;
said apparatus including:
a radiant energy transducing element having a surface shape
incongruent to the cross-sectional shape of a single said space;
and
a radiant energy guide structure for congruence matching,
positioned between said file and said element and having first and
second ends respectively adjacent said element and the periphery of
said file; said first end being shaped congruent to the shape of
said element and said second end being shaped congruent to said
cross-sectional shape of a single said space.
6. Space locating apparatus in accordance with claim 5 wherein:
said radiant energy is light energy;
said transducing element is a photodetector having a generally
circular shaped sensing surface considerably wider in diameter than
the width of a space to be sensed; and
said guide structure includes a fiberoptic array shaped congruent
to the shape of said photocell at said first end and shaped in a
linear array, congruent to the cross-sectional shape of a single
said space at said second end.
7. Space locating apparatus in accordance with claim 6 including a
shroud member positioned between said discs and said fiber optic
array; said shroud member having an enclosed light confining
passageway for congruently coupling light from an adjacent one of
said disc spaces to said line-shaped end of said fiber array with
minimal loss of light due to the curvature of the discs and minimal
increment of stray light from sources other than said one disc
space.
8. Space locating apparatus in accordance with claim 7, wherein
said shroud comprises an arcuately shaped light receiving end
conforming closely to the path of revolution of the discs for
coupling light from said one space between said discs and
minimizing escape of light from said one space to the space
external to said shroud passageway.
9. Space locating apparatus in accordance with claim 8, wherein
said shroud and at least the line-shaped end of said fiber optic
array are mounted for movement as a unit parallel to the axis of
rotation of said discs.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention pertains to photoelectric apparatus for counting
interface gaps between spaced storage discs in a rotary file of
multiple flexible or floppy type magnetic discs in continuous
motion; especially when the disc edges are subject to transient
fluttering parallel to the axis of rotation.
2. Cross References to Related Applications
1. U.S. patent application Ser. No. 375,985 by R. O. Cobb et al,
filed July 2, 1973 now U.S. Pat. No. 3,835,998.
2. U.S. patent application Ser. No. 414,614 by R. J. Penfold et al,
filed Nov. 7, 1973, now U.S. Pat. No. 3,867,723.
3. Description of the Prior Art
The above cross-referenced Penfold et al application discloses a
rotating magnetic disc storage file, including multiple flexible or
floppy storage discs maintained in continuous high speed rotation,
wherein successive discs are separated by relatively small diameter
thin spacer discs. Typically the storage discs have nominal
thickness of about 0.003 inch. Outward application of pressured
air, through the longitudinally apertured spindle which supports
the discs, causes the storage discs to be spaced uniformly at
intervals of about 0.003 inch when rotating at typical speed of
1800 rpm.
Spaced disc files of this type represent improvement, in terms of
design simplicity, operational stability and access time
efficiency, over flexible disc files of the kind disclosed in U.S.
Pat. Nos. 3,509,533 (Krijnen) and 3,618,055 (Van Acker). In turn
the latter files represent ostensible evolutionary advancements
over an earlier type of non-flexible (or semi-flexible) disc file
disclosed in U.S. Pat. No. 3,130,393 (Gutterman). In the Gutterman
patent relatively thicker slidably mounted magnetic discs rotate
contiguously in a closed air-tight environment and are separated
for access by application of high pressure air radially inward at a
selected interface. This displaces a subset of the discs, in a
piston-like operation, to form a space (gap) suitable for
accommodating a magnetic head.
In files of stacked flexible discs it is usually desirable to
locate the access position by sensing and counting operations.
However such sensing and counting must not be subject to error when
there is fluttering of the discs or run-out variations in disc
thickness which could cause double sensing (false counting) of a
gap.
The Gutterman patent specifically suggests cumulative electronic
counting of disc edges sensed by electromechanical (piezoelectric)
means and inplies alternative use of magnetic or optical edge
sensing.
The above cross-referenced patent application by R. O. Cobb et al
addresses problems incidental to tracking axial components of
flexible disc motion by parallel optical sensing. It discloses a
stationary array of integrated circuit photodetectors which is
light coupled in parallel to all of the discs. Individual
photodetectors of the array have width dimensions an order of
magnitude smaller than the nominal disc thickness so that several
detector elements are light coupled to each disc. By sensing the
distribution of light across the detector elements the position of
the access apparatus relative to the discs is instantaneously
distinguishable for parallel sampling and electronic counting. With
large numbers of discs this arrangement can be quite costly, at the
present state of development of integrated circuit technology, and
dissipates more power than the serial sensing arrangement suggested
by Gutterman which would use a single sensor coupled to only one
disc interface at a time.
However, in sensing the discs one at a time by optical coupling,
light must either be confined to one disc edge and sensed upon
reflection or confined to one interface space (gap) and sensed
after transmission. Realization of this under fast access
conditions with a high degree of reliability is not simple to
achieve. For reflective coupling optimal results are realized only
if the disc edges are processed to a smooth reflective finish
(e.g., by lathe trimming, polishing, painting, etc.) and this
naturally increases system fabrication and maintenance costs. On
the other hand transmissive coupling through inter-disc spaces
presents difficulties due to the narrow width (about .003 inch and
long length of the coupling path (several inches in a file of 12
inch diameter discs). Light attenuation due to dispersion and
scattering (e.g., from dust, debris on the disc surfaces, and/or
edge slivers on the discs) weakens the coupling and thereby
increases the possibility of erroneous readings. Upon error the
access assembly must be repositioned degrading access time
performance.
With either type of coupling (reflective or transmissive) transient
light-to-dark transitions due to transitory components of disc edge
motion (due to fluttering and/or variations in disc shape or
thickness) can be quite difficult to track and accurately
count.
Transmissive coupling losses can be reduced by use of coherent
(i.e., laser) light but the increased cost may be unattractive.
Another obvious alternative, increasing the intensity of
non-coherent source light, presents the risk of subjecting the
recording surfaces of the discs to possible damage or warping
stress due to the accompanying heat.
SUMMARY OF THE INVENTION
An object of the present invention is to provide economical, safe,
rapidly responsive and reliable sensing apparatus for accurately
counting and tracking gaps between discs in files of spaced
flexible discs of the kind disclosed in the above cross-referenced
Penfold et al application.
Another object is to provide counting and tracking apparatus, for
controlling rapid access to densely configured flexible discs
rotating cylindrically at high speeds, based upon sensing of safe
intensity levels of non-coherent light coupled through spaces
between the discs.
Another object is to provide safe, accurate, efficient, reliable
and economical disc gap sensing apparatus for serially counting
disc gaps in a cylindrically rotating file of spaced flexible discs
subject to transient axial motion.
A feature of the invention is the employment of specifically
arranged offset light guiding and shaping elements for coupling
light efficiently from narrow gap spaces between rotating flexible
discs to a relatively "wide" surface.
Another feature of the invention is the employment of a pair of
slightly offset sensing assemblies, each assembly comprising a
light confining shroud and fiber optic shaping array in congruent
association with respective photocells and tracking logic circuitry
for providing accurate counting and tracking of disc gaps in a
rotary disc file of spaced flexible discs. A necessary condition is
that the assembly offset is less than the nominal gap width (a
sufficient condition is that the offset be less than half the
nominal gap width).
A feature of the tracking and counting logic is the ability to
avoid posting of false counts due to transitory axial motion of the
disc edges.
The foregoing and other objects, features and advantages of this
invention will be more fully appreciated upon consideration of the
following detailed description and claims.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic end view of a dynamically rotating
cylindrical disc file of flexible discs, and basic sensing
apparatus for sensing spaces (gaps) between the discs.
FIG. 2 is a schematic providing perspective views of the shroud,
optical fiber array and photodetector elements of the basic sensing
apparatus disclosed in FIG. 1.
FIG. 3 is a side elevational view of a dual sensing configuration
in accordance with the invention.
FIG. 4 is a sectional view along lines 4--4 in FIG. 3.
FIGS. 5-7 are schematics of counting and tracking logic circuits
coupling to the photocells of FIG. 3.
FIG. 8 is a signal waveform diagram for explaining operation of the
counting and tracking circuits of FIGS. 5-7.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT
Referring to FIG. 1 a multiple disc file assembly of the type
described in the above cross-referenced Penfold et al application
includes a plurality of spaced flexible discs 1 and inter-disc
flexible spacers 2 continually revolving about axis 3. Sensing
apparatus in accordance with the present invention includes light
source 5, collimating lens 7, apertured shroud 9, light shaping
fiber optic unit 11 and light sensor 13. The sensing apparatus is
mounted upon a not shown carriage for movement parallel to the
rotational axis 3 of the disc assembly so that the light source 5,
7 and sensing assembly 9-13 become alternately coupled and
decoupled through the spaces (gaps) between the discs as the
sensing apparatus moves relative to the discs; whereby electrical
variations produced in the sensor 13 may be cumulatively
counted.
Not shown access apparatus (including a magnetic head), located
beneath and in line with the shroud 9, is transported together with
the sensing assembly 9-13; whereby the access apparatus can enter
and displace the discs at randomly preselected gaps when the
cumulative count corresponds to a selected address and thereby
provide recording/reproduction access to the disc surfaces.
In a typical environment the discs would have nominal thickness of
about 0.0017 inch and nominal separation spacing of approximately
0.003 inch. In this environment shroud 9 provides a slotted light
coupling passageway, of rectangular cross-section, about 1/4 inch
long by 0.003 inch wide. The light receiving end of the shroud is
made to conform closely to the disc edges along the path of disc
rotation.
Light shaping unit 11 preferably consists of an array of light
conducting fibers (nominal fiber thickness 0.003 inch) arranged in
a line-to-spot shaping configuration. The lineshaped light
receiving end of the fiber array is aligned with and substantially
congruent in shape to the light emitting end of the shroud. The
spot-shaped light emitting end of the fiber array is aligned with
and shaped substantially congruent to the sensitive light receiving
surface of sensor 13. Sensor 13 has a circularly shaped sensing
surface approximately 0.03 inch in diameter (approximately ten
times the width of the disc aperture). The area of the sensing
surface (about 0.0007 square inch) is approximately equal to the
area of the shroud aperture at the fiber interface.
Shaped fiber arrays of this type are formed from planar sheets of
optical fibers bound by an adhesive. In wellknown fashion the sheet
is cut to required dimensions, the adhesive is removed from the end
which is to be shaped to spot form and the separated fibers are
rearranged and readhered in spot formation.
In the preferred embodiment next described with reference to FIGS.
3-8 a dual offset configuration of the light receiving parts of the
above-described basic sensing apparatus is utilized. The shroud
comprises two separate 1/4 inch by 0.003 inch light guiding
passages 9a and 9b preferably lined with light reflective material
and may be formed in sections with the passages milled as groves in
the sections. The milled passages are parallel but relatively
offset from each other by a displacement d (FIG. 4) which is less
than the width of a disc gap (suitably d = 0.0010 inch for discs
nominally 0.0017 inch thick). Interfacing congruently with the
shroud exit apertures are relatively offset linearly configured
ends of respective fiber optic shaping arrays 11a and 11b (each 1/4
inch by 0.003 inch). Arrays 11a and 11b have "spot" configured
output ends interfacing congruently with respective photodetectors
13a and 13b.
The shroud/fiber array assembly is secured to housing 15 (FIG. 3)
by not shown screws. Light source 5 (FIG. 3) is also secured to the
housing. The housing is mounted upon a not shown carriage for
movement parallel to the axis of rotation of the discs.
The photo reflectors 13 may conveniently be Texas Instruments Co.
type LS 654 light sensors. Light source 5 may be a Welch Allyn
Incadescent Lamp (5V, 4w) type No. 999079-48, collimated by a
simple plano convex lens.
Referring to FIGS. 5-8, outputs of photodetectors 13a and 13b are
channelled through respective amplifiers 15a and 15b, to
differential comparator circuits 17-1, 17-2, 17-3 and 17-4 (FIG.
5). Amplifiers 15a and 15b are 30-db non-inverting operational
amplifiers with pre and post voltage follower stages. Each
amplifier stage and follower stage may be an RCA type CA3030A
integrated circuit amplifier. Each of the comparators 17 is a
standard differential voltage comparator circuit; for instance,
Texas Instrument SN 72810.
Outputs of comparators 17 are inverted by respectively numbered
inverters 19-1, 19-2, 19-3 and 19-4. The comparator outputs are
designated by the respective comparator numbers (17-1, 17-2, 17-3,
17-4) and the inverter outputs are designated by the respective
comparator number overscored (17-1, 17-2, 17-3, 17-4). Comparator
17-1 compares outputs of amplifier 15b with the offset output of
amplifier 15a attenuated by the small forward voltage drop across
diode 21-1. Similarly, comparator 17-2 compares output of amplifier
15a with offset output of amplifier 15b attenuated by forward
voltage drop across diode 21-2. Characteristics of diodes 21-1 and
21-2 are chosen to provide transitional comparator outputs when
outputs of amplifiers 15a and 15b are slightly offset from one
another and are passing through a "centered" transitional phase
corresponding to centered positioning of apertures 9a, 9b slightly
left and right of a disc gap center line. The circuit parameters
are chosen so that the output of 17-1 is at a low or negative level
when the center of apertures 9a and 9b is to the right of a fine
left limit position just left of the center line of the disc gap
coinciding with the rise of 17-1 after the gap center line at 22-1
(FIG. 8). This threshold position is a fraction of a millimeter
(i.e., a fraction of a gap width) to the left of the gap center due
to the relatively small voltage drop across the diode. Similarly,
circuit parameters associated with comparator 17-2 and diode 21-2
are designed so that the output of 17-2 is negative or low when the
center of apertures 9a, 9b is to the left of a fine limit position
just to the right of the gap center line (by a fraction of the gap
width) coinciding with the rise of 17-2 at 22-2 (FIG. 8).
Comparators 17-3 and 17-4 are referenced to voltage V to provide
for respective comparator outputs to be low or negative so long as
the center of apertures 9a, 9b is respectively to the right and
left of respective left and right coarse limit positions (FIG. 8)
relative to a gap center line. Thus, comparators 17-1 and 17-2
provide outputs capable of indicating that the aperture center is
aligned with the center line of a facing disc gap within limits of
a fine or narrow range and comparators 17-3 and 17-4 provide
outputs indicating that the aperture center is aligned with the
center of a facing disc gap within limits or a coarse or wide
range. By virtue of the aperture offset these comparators provide
sequential transitions, indicative of the direction of movement of
the apertures relative to the above-mentioned threshold limits,
which are useful to prevent false counting of disc gaps when the
disc edges are fluttering in a random manner.
Outputs of comparators 17-1 through 17-4 control AND circuits (A)
of FIG. 6 which condition latches 23-1, 23-2, 23-3, 23-4, 23-5,
23-6 (FIG. 6) under conditions determined by the instantaneous
output conditions of these comparators and latches. Outputs of
comparators 17-1 through 17-4 are also applied in combination with
outputs of latches 23-1 through 23-6 to centering logic (FIG. 7).
The centering logic operates to provide drive right and drive left
control signal functions for servoing the aperture system 9-11
(FIG. 3) to the narrow centering range of a selected disc gap under
conditions described below.
Outputs of count latches 23-2 and 23-5 are applied respectively to
decrement and increment inputs of forward-backward counter 25.
Outputs of counter 25 are compared by vector comparator circuit 27
to contents of an address register 29. Comparator 27 provides
output indication of equality (Address Compare) or inquality (Not
Address Compare) between the compared address and count arguments.
With the inequality indication comparator 27 provides high or low
output indication indicating that the address argument is greater
or less than the count argument. The high-low indications are used
for servoing the sensing system 9-13 (FIGS. 1-3) towards the gap
corresponding to the address set into register 29.
Comparator 27 is of the type described, for instance, in "The Logic
Design of Transistor Digital Computers", by G. A. Maling and J.
Earle, Prentiss Hall, 1963, pages 262-264.
The general operation of the circuitry just described is as
follows. When vector compare circuit 27 detects inequality between
the compared address and count arguments it provides high or low
output indication to respective AND circuit 31-1 or 31-3 (FIG. 7)
to provide right or left drive impetus to the not-shown servo motor
which positions the sensing assembly 9-11. This drives the assembly
9-11 in a direction tending to reduce the difference between the
compared address and count arguments until equality is detected by
vector compare circuit 27. At equality (Address Compare) AND
circuits 31-2 and 31-4 are conditioned to pass right and left drive
impetus to the above-mentioned servo motor which drives the sensing
assembly in a direction tending to maintain centering of the
sensing apparatus within the narrow range limits to either side of
the center of the selected disc gap.
Additional details of operation of the foregoing circuits are now
described with reference to FIGS. 3-8. Assume that the disc gaps
are numbered in descending order, in a direction into the page in
the view of FIG. 3, so that the furthest gap would have address
number 1, the next furthest number 2, etc. Assume also that the
count in counter 25 is normally progressively incremented for left
movement of the sensing assembly relative to the disc gaps (up in
FIG. 3) and decremented for right movement relative to the disc
gaps (down in FIG. 3).
Disregarding transitory motion of the disc edges left relative
movement of aperture 9a, 9b causes Add Inhibit Latch 23-6 and
Subtract Inhibit Latch 23-3 to be set and reset in specific phase
sequence so as not to interface with setting of Add Latch 23-4
while inhibiting setting of Subtract Latch 23-1 (refer to positions
37, 39, 41 FIG. 8). In turn this allows Add Count Latch 23-5 to be
set in coincidence with action 22-4 (FIG. 8) just as the right
limit of coarse centering range is passed while it blocks earlier
setting of the Subtract Count Latch 23-2. Setting of Add Count
Latch results in incrementing of counter 25 (FIG. 7). Add Latch and
Add Count Latch are coincidentally reset just as the aperture
center passes into fine centering range coincident with the rise of
centered position 43 (FIG. 8). Thus if there were no relative disc
edge motion the count would be monotonically increased until it
matched the selected address (Address Compare).
Disregarding transistory disc motion right movement of the aperture
center relative to the disc gaps causes counter 25 to be
progressively decremented. Subtract Inhibht Latch 23-3 is set
before the aperture center reaches the left limit of the coarse
centering range and reset just as this limit is passed. Reset state
of Subtract Inhibit Latch allows Subtract Latch 23-1 to be set
before the left coarse limit is reached, which in turn allows
Subtract Count Latch 23-2 to be set just as the left coarse limit
is reached. Setting of Subtract Count Latch decrements counter 25.
Subtract Latch and Subtract Count Latch are reset in coincidence
with the left rise of "centered" at the left limit of "fine"
centering. Thus the counter is progressively decremented as the
aperture moves to the right relative to the disc gaps.
Upon reversal of apparent aperture motion due to transitory axial
motion of the disc edges (due for instance to fluttering, thickness
run-out, etc.) the effect on the accumulated count in counter 25
will depend upon the aperture position at the instant of reversal.
If the aperture is actually moving to the left and ostensibly
reverses movement due to a left shift of the disc edges the
increment controls will not be affected if the reversal occurs
while the Add Inhibit Latch is set (i.e. before a count increment
is posted) and the decrement control logic will be prepared as for
a straightforward decrement operation. If reversal occurs after
setting of Add Count Latch (i.e. after posting of a count
increment) Add Count Latch is reset either at position 41 or at the
right limit of fine centering and the control logic (i.e. the
inhibit latches) and conditioned to allow decrementing operation
while inhibiting incrementing. A decrement is then posted only if
the aperture center passes into the centering range of the next gap
to the right of the gap for which count increment has just been
posted. Upon a second reversal of the apparent motion to the
original left direction, after posting of the count increment but
before a decrement has been tallied, the Inhibit logic remains
conditioned to prevent setting of Add Count Latch and thereby
prevent posting of a false count increment when the aperture center
passes the center of the previously counted gap. By similar
considerations it may be verified that under all conditions only
one net count is tallied per gap regardless of the transient motion
of the disc edges.
At Address Compare the centering logic associated with latches 51
and 53 (FIG. 7) in cooperation with AND's 31-2 and 31-4 (FIG. 7)
supplies left and right drive impetus to servo the aperture center
to the center of the selected gap. As the aperture center initially
enters coarse centering range of the selected gap from the right or
left Subtract Count Latch or Add Count Latch is respectively set.
In addition to giving rise to the final count decrement or
increment producing the Address Compare indication, this enables
the respective latch 53 or 51 to be set and thereby supply
sufficient impetus to carry the aperture center through the center
line of the selected gap. Thereafter, as the aperture center passes
out of fine centered range, either to the left or right of the
selected gap, the appropriate latch 53 or 51 is set to provide
reverse right or left drive stimulus to the servo motor returning
the aperture center to the centered position (note positions 45,
FIG. 8).
Logic 55 (FIG. 7) receives outputs of AND's 31-2 and 31-4 of FIG. 7
representing centering of the sensing apparatus at the selected gap
and produces an Insert signal which is used to disable AND'S 31-1
through 31-4 and to control movement of not-shown access apparatus
into the selected gap. This separates the rotating discs at the
gap, in order to make room for operation of a transducing head
relative to the disc surfaces facing the gap. Disabling of AND's
31-1 through 31-4 blocks drive impetus to the sense assembly servo
motor preventing sense assembly movement until access apparatus has
been removed from the gap. Upon removal of the access apparatus
from the gap the Insert signal is removed, re-enabling AND circuits
31-1 through 31-4 to re-supply drive stimulus to the sense assembly
servo motor. A new address may be set into register 29 (FIG. 7) at
any time after rise of the Insert command since the servo drive
controls 31-1 through 31-4 are then effectively disabled.
If the disc edges are consistently fairly stable Insert controls 55
may be made simply responsive to OR'd outputs of AND's 31-2 and
31-4 for setting a latch which provides the Insert and Insert
conditions and that latch may be reset when the access operation is
terminated by removal of the access assembly. On the other hand, if
there is extensive instability in the system or if the discs are
tightly spaced, or if the reaction time of the access apparatus is
limited, the OR'd output of AND's 31-2 and 31-4 may be subjected to
filtering in order to determine a sufficiently stable condition for
guaranteeing accurate placement of the access apparatus into the
correct disc gap.
While the invention has been particularly shown and described with
reference to a preferred embodiment thereof, it will be understood
by those skilled in the art that the foregoing and other changes in
form and detail may be made therein without departing from the
spirit and scope of the invention.
* * * * *